linux-kernel - Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets

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Message-ID: <20220510165233.yahsznxxb5yq6rai@skbuf>
Date:   Tue, 10 May 2022 19:52:33 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Felix Fietkau <nbd@....name>
Cc:     Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        DENG Qingfang <dqfext@...il.com>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets

On Tue, May 10, 2022 at 04:52:16PM +0200, Felix Fietkau wrote:
> 
> On 10.05.22 14:37, Vladimir Oltean wrote:
> > On Tue, May 10, 2022 at 11:40:13AM +0200, Felix Fietkau wrote:
> > > Padding for transmitted packets needs to account for the special tag.
> > > With not enough padding, garbage bytes are inserted by the switch at the
> > > end of small packets.
> > 
> > I don't think padding bytes are guaranteed to be zeroes. Aren't they
> > discarded? What is the issue?
> With the broken padding, ARP requests are silently discarded on the receiver
> side in my test. Adding the padding explicitly fixes the issue.
> 
> - Felix

Ok, I'm not going to complain too much about the patch, but I'm still
curious where are the so-called "broken" packets discarded.
I think the receiving MAC should be passing up to software a buffer
without the extra padding beyond the L2 payload length (at least that's
the behavior I'm familiar with).